The present invention relates generally to serial radiographic systems and more particular to an improved X-ray film changer and method for storing and transporting X-ray film packs.
In serial radiography, a series of radiographs are produced by sequentially feeding a series of X-ray films, one at a time, to an exposure station. The exposure station is aligned with a source of X-rays which directs a beam of X-rays through the exposure station for exposing the films.
High-speed X-ray systems expose films at frame rates of up to six per second. High speed in radiography is desirable not only for minimizing the duration of the examination during which the patient must be immobile, but more importantly, for assuring that a timely exposure will accurately record an event of short duration.
Certain vascular studies are examples of medical diagnostic techniques which require high-speed exposures at high frame rates. With such a study a fluid which is relatively opaque to X-rays is injected into a patient's blood stream. Rapid sequence radiographs produce diagnostically useful information about the patient's blood vessels. This radiographic sequence must be rapid because the opaque material passes very quickly through that portion of a patient that is under investigation.
This high-speed requirement has posed problems in developing an optimized system. Such a system should be characterized: (a) by a minimum of system vibration, (b) by a minimum of noise, (c) by a "see through" exposure station allowing unobstructed fluoroscopy, (d) generally as an efficiently compact system which does not require expensive and bulky vacuum generating apparatus or mechanisms for opening evacuated pouches, (e) by assurance that there is intimate film contact with a pair of intensifying screens which usually accompany the film, and (f) avoiding any possibility of a film being scratched or otherwise damaged during transport or exposure.
One problem encountered in high-speed serial radiography has been the mechanism for insuring exposure of the X-ray film with the proper radiation energy level. The X-ray film generally has emulsion which is sensitive to both light and X-ray radiations on both surfaces of a substrate. In order to minimize patient exposure to radiation, a radiograph is typically made with a pair of light emissive intensifier screens in contact with the X-ray film. The intensifying screens emit light in response to impinging X-rays for exposing the film more quickly than it can be exposed by X-rays alone. Use of intensifying screens thus provides an increased intensity of electromagnetic radiation for exposing the X-ray film which reduces both the time required to produce the radiograph and the patient's exposure to X-radiation.
For maximum resolution the intensifying screens must be maintained in uniform intimate engagement with the film during X-ray exposure. With conventional radiographic procedure, an X-ray film and a pair of screens are inserted in a light-tight film container, known as a cassette. The cassette protects the X-ray film from exposure to light, dust and abrasions generated during handling of the film and is intended to maintain the intensifying screens in the required intimate contact with the film.
Initial proposals suggested use of springs and pressure plates in the cassette to maintain the intensifying screens in contact with the film. Later proposals suggested cassettes which utilized vacuum actuated pressure plates to eliminate the springs. More recently heat sealable, X-ray transparent pouches have been used for containing the film and the screens. The intensifying screens and X-ray sheet are inserted into a pouch and then the pouch is evacuated and sealed. Engagement of the film and screen is maintained by atmospheric pressure. Vacuum pouches which have been used successfully are described in detail in U.S. Pat. No. 3,348,042, entitled, FLEXIBLE LIGHT IMPERVIOUS FLUID TIGHT X-RAY FILM PACK, (here the FILM PACK patent) issued to Umberg, et al.
The evacuated, X-ray transparent pouch of the FILM PACK patent provided an improved and reliable cassette for holding the film and the screens in engagement. While the pouches have enjoyed outstanding commercial success, there are certain characteristics which become, when compared to the present invention, disadvantages. These include:
a. The loading, evacuating and sealing of the pouches is relatively slow;
b. Auxiliary equipment is required for generating the vacuum;
c. The pouches are not reusable after the film has been exposed because the bags must be cut open to remove the exposed film for processing; and,
d. When made for use with a film changer the pouches are both bulky and heavy. The bulk and weight of the pouches makes the transport of pouches to and from an X-ray room relatively difficult.
A serial radiographic system adapted especially for using film cassettes of the evacuated pouch type is described in U.S. Pat. No. 3,560,743, entitled, X-RAY FILM CHANGER HAVING A STORAGE CHAMBER WITH A WALL WHICH PIVOTS AS CASSETTES ARE FED THERETO, (here the FILM CHANGER patent) issued to Smit, et al. The FILM CHANGER patent discloses and claims a film changer which has a loading magazine positioned under an X-ray source. In use, a plurality of the vacuum sealed pouches are stacked in the loading magazine. After the film in the top cassette is exposed, the cassette is stripped from the stack to allow exposure of that film in the next underlying cassette. Each pouch has, in addition to the sheet of X-ray film and the intensifying screens, a lead backing sheet and a filler sheet. The backing sheets prevent exposure of the underlying film sheets when the top film sheet is exposed.
Film changers of the type described in the FILM CHANGER patent have proven to be reliable and acceptable, but when compared to the changer of this invention, they have inherent disadvantages. The lead backing sheets are relatively heavy and increase the mass of the film cassettes. The heavier film cassettes require a heavier, more massive system for stripping a cassette with an exposed film from the load magazine. The lead backed cassettes are also bulky requiring relatively large supply and collection magazines.
Another disadvantage of most prior changes is that they have been noisy. Noise in a radiographic system is highly undesirable especially with a high-speed changer. A sick patient may suffer discomfort due to pain or an unstable emotional state. Undue noise during an examination may aggravate the patient's discomfort especially where his head is positioned adjacent the changer for cranial studies.
A design goal of recent radiographic systems also has been the achievement of a system which provides the capability optionally to conduct fluoroscopic studies of a patient. Some high-speed film changer designs did not allow both a fluoroscopic study and a radiographic study. One such system was the film changer described in the FILM CHANGER patent where the supply magazine for storing cassettes with unexposed film was located directly behind an exposure station. This location of the loading magazine prevented use of an image tube behind the exposure station.
Another serial radiographic system attempted to provide fluoroscopic capability by offsetting its film supply from its exposure station and providing an image tube behind the exposure station. Radiographs were made by transporting sheets of X-ray film in sequence from the supply to the exposure station where the film was clamped between a pair of intensifier screens mounted at the station.
With some of these systems fluoroscopic operation was somewhat limited by apparatus which was required for mounting the intensifier screens since such apparatus could, to some extent, obstruct the X-ray path. More importantly, since the intensifier screens absorb radiation and since the screens are between the X-ray source and the image tube, they function as filters and, as a consequence, fluoroscopic radiation levels are needlessly high.